The present invention relates to a solder powder to be used together with a flux in a solder paste (soldering paste). More particularly, the present invention relates to a method for preparing such a solder powder, and to a solder paste including a solder powder and a flux.
As is customary in this field, throughout this specification, for example xe2x80x9c63Snxe2x80x94Pbxe2x80x9d means a composition of 63 wt % Sn and Pb for the remaining portion, and for example xe2x80x9cSn-8Zn-3Bixe2x80x9d means a composition of 8 wt % of Zn, 3 wt % of Bi, and Sn for the remaining portion.
Methods for applying solder to electronic parts include, for example, soldering with a soldering iron, immersing, and reflow soldering (referred to as xe2x80x9creflowingxe2x80x9d below). For the surface mounting of for example IC chips, reflowing is used, because bridges between the leads of the surface-mounted parts do not develop easily, and this method is superior to other methods with regard to productivity. In reflowing, a solder paste including a mixture of a powder of a solder alloy and a flux usually is printed on certain soldering positions using a metal mask or a silk screen, heated in a reflow furnace, thereby applying the solder.
For soldering by reflowing, a Snxe2x80x94Pb alloy is commonly used. Since Snxe2x80x94Pb alloys have the lowest melting point as an eutectic composition (which is 183xc2x0 C. for 63Snxe2x80x94Pb), the soldering temperature is comparatively low at about 220 to 230xc2x0 C. Therefore, the danger of damaging the electric parts by heat is small. Moreover, Snxe2x80x94Pb alloys have very good soldering properties. However, since these solder alloys contain Pb, there is considerable concern that the Pb that is washed out of disposed electric appliances contaminates the groundwater and reaches the human body, for which it is a serious health threat. For this reason, Snxe2x80x94Ag alloys, Snxe2x80x94Sb alloys, Snxe2x80x94Bi alloys, and Snxe2x80x94Zn alloys have been proposed as lead-free solder alloys.
Among Snxe2x80x94Ag alloys, an eutectic composition of Sn-3.5Ag has the lowest melting point, which is 221xc2x0 C. With this composition, the soldering temperature will be as high as 260-270xc2x0 C. Among Snxe2x80x94Sb alloys, a composition of Sn-5Sb has the lowest melting point, which is 235xc2x0 C. in the solidus line and 240xc2x0 C. in the liquidus line. For this composition, the soldering temperature is 280-300xc2x0 C. which is even higher than for Sn-3.5Ag. For Snxe2x80x94Bi alloys, the eutectic temperature of eutectic Sn-58Bi is 139xc2x0 C. This eutectic temperature is sufficiently low, but Snxe2x80x94Bi alloys are brittle and hard, so that they result in inferior mechanical properties such as the tensile strength of the soldered parts.
For Snxe2x80x94Zn alloys, the eutectic temperature of eutectic Sn-9Zn is 199xc2x0 C. This eutectic temperature is close to the eutectic temperature of 183xc2x0 C. for conventional 63Snxe2x80x94Pb eutectic solder. Moreover, Snxe2x80x94Zn alloys have excellent mechanical properties.
However, there are also deficiencies in the soldering properties of Snxe2x80x94Zn alloys. To improve the soldering properties of Snxe2x80x94Zn alloys and increase its mechanical strength, it has been proposed to add a suitable amount of for example Ag, Cu, Bi, In, Ni, or P to the Snxe2x80x94Zn alloy to obtain a Snxe2x80x94Zn-based solder alloy (see for example JP-A-9-253882).
Moreover, since the reactivity of the Zn in the Snxe2x80x94Zn alloy is strong, neighboring solder particles cluster together easily, so that the viscosity of the solder rises when used as a paste. If the viscosity of a solder paste becomes too large, the printing of the paste on the board becomes difficult. Since the surface of the Zn in the Snxe2x80x94Zn-based alloy oxidizes easily, the melting point of the alloy tends to increase as well. It has been proposed to lower the viscosity of the paste by increasing the amount of activator (for example an amine-halogenated salt of a hydroacid) in the flux used together with the Snxe2x80x94Zn-based solder alloy.
JP-A-9-327789 proposes an improved flux for suppressing a change of the paste viscosity over time. The flux that is disclosed in this publication includes an organic acid such as malic acid or tartaric acid, which includes a carboxyl group and a hydroxyl group. These organic acids suppress the reaction between Zn and other polymers included in the flux components, thereby relaxing the increase of the flux viscosity. This flux also includes an organic component such as ester phthalate or sorbitan fatty acid ester. These organic components suppress reactions of the Snxe2x80x94Zn-based alloy by adhering to the surface of the solder alloy.
JP-A-215884 attempts to coat a powder of the solder alloy itself before mixing it with the flux. The specific method disclosed in this publication is to immerse a powder of Bi-43Sn solder alloy of 10 to 45 xcexcm particle size in an aqueous solution including a corrosion inhibitor such as a benzothiazol derivative, an amine, or a thiourea.
However, as is disclosed in JP-A-9-253882, even when the characteristics of the Snxe2x80x94Zn-based solder powder were improved by adding small amounts, the soldering properties for the reflow method were still insufficient. In particular, there was the problem that copper foil land portions on the printed circuit board remained without wetting the soldering portions completely. There was also the problem that the solder paste flowed from the copper foil portions between the semiconductor leads, and adhered to the printed circuit board in form of solder balls.
If the amount of the activator in the flux is increased to lower the viscosity of the paste, then the problem results that the activator remaining after the soldering process whitens because of age deterioration, and the strength and conductivity of the solder connection are lowered.
As explained in JP-A-9-327789, the contact between the activator component in the flux and the solder powder cannot be prevented effectively by coating the solder alloy particles with components in the flux.
JP-A-8-215884 discloses specifically only a Snxe2x80x94Bi alloy, and does not disclose any material or method for preventing the thickening of a solder paste including a highly reactive metal, such as Zn.
With conventional methods, such as the ones mentioned above, the change of the Snxe2x80x94Zn-based solder alloy over time cannot be suppressed sufficiently. If the thickening cannot be suppressed sufficiently, the so-called pot life is shortened, and the problem results that when applying the solder paste to a circuit board by screen printing, it becomes difficult to print the solder powder through the mesh and the holes in the mask tend to clog.
It is an object of the present invention to provide a solder powder including Sn and Zn, which has good soldering properties, and wherein reactions between the activation component in the flux and the alloy component can be suppressed. It is a further object of the present invention to provide a method for preparing such a solder powder. It is a further object of the present invention to provide a solder paste including Sn and Zn, having good soldering properties, whose change over time can be suppressed.
To achieve these objects, a solder powder of the present invention, which constitutes a solder paste together with a flux, includes Sn and Zn, wherein a salt of an organic acid is adhered to a surface of the solder powder.
Even when the solder powder of the present invention is used together with a flux, a reaction with the activation component in the flux can be prevented, while maintaining good soldering properties.
A first solder paste according to the present invention includes the above-noted solder powder and a flux. A second solder paste according to the present invention includes a solder powder and a flux, wherein the solder powder includes Sn and Zn, and the flux includes 0.5 to 10 wt % of a nonionic surfactant.
With the solder paste of the present invention, a change of the solder paste over time can be prevented, while maintaining good soldering properties.
A method for preparing a solder powder in accordance with the present invention includes preparing a solution saturated with a salt of an organic acid, and contacting the salt of an organic acid with a solder powder comprising Sn and Zn so as to adhere the salt of an organic acid to a surface of the solder powder.
Throughout this specification, a xe2x80x9csolution saturated with a salt of an organic acidxe2x80x9d means a solution containing the soluble limit (saturated solution) or more (oversaturated solution) of a salt of an organic acid.